1. Field of the Invention
The present invention relates to a phase shift mask and a method for fabricating the same, and more particularly to a phase shift mask capable of sequentially phase-shifting the incident light three times near its light shield film pattern, thereby utilizing an interference effect between two electric fields.
2. Description of the Prior Art
The recent trend to fabricate semiconductor devices having a light, thin, simple and compact structure results in a reduced distance between adjacent wirings, an increased topology and a reduced size of unit elements such as transistors or capacitors. For this reason, the requirement to form photoresist film patterns with a micro dimension has been increased.
Generally, light exposure masks, which are used in the light exposure process for forming photoresist film patterns, are fabricated by coating a light shield film comprised of a chromium layer or aluminum layer over a quartz substrate, and then etching the light shield film in accordance with an ion beam etching method, thereby forming a light shield film pattern. However, with such general light exposure masks it is difficult to form patterns with a micro dimension smaller than the limit of the light resolution of steppers. Furthermore, with conventional photoresist solutions and steppers, for example, G-line steppers with the wavelength of 436 nm or I-line steppers with the wavelength of 365 nm, it is difficult to form patterns having a micro dimension of 0.5 .mu.m or below.
On the other hand, semiconductor devices having a high integration degree of 64 Mega DRAM grade or greater need a micro pattern with a dimension of 0.5 .mu.m or below. In order to form such a micro pattern, phase shift masks capable of forming photoresist film patterns exhibiting a high resolution have been used.
Typically, such phase shift masks have a phase shift film pattern adapted to shift the phase of light through an angle of 180.degree. to 90.degree. along with light shield film patterns so as to uniformly maintain the amplitude of light irradiated onto a wafer in the light exposure process. In other words, such a phase shift mask utilizes the principle of minimizing the light exposure effect resulting from the interference between the light passing through the phase shift film pattern and the light passing through a pattern disposed adjacent to the phase shift film pattern, thereby improving the resolution of a photoresist film pattern finally formed.
The phase shift film pattern is made of a phase shift material exhibiting a refractivity of n to have a thickness capable of shifting the phase of light with a wavelength of .lambda. through an angle of 160 to 200.degree. so that the light irradiated onto a photoresist film can exhibit an increased contrast ratio. For example, for G-line or i-line incident light, the phase shift mask may have 3,400 to 4,000 .ANG. when it is comprised of a spin-on-glass (SOG) film, oxide film or nitride film.
Where such phase shift masks are used, it is possible to form micro patterns having a dimension of 0.5 .mu.m or below using conventional photoresist solution and steppers. Such phase shift masks are classified into the Levenson type and the edge emphasis type.
An example of a conventional phase shift mask will now be described in conjunction with FIGS. 1 to 3.
Referring to FIG. 1, a phase shift mask 2 is shown, which includes a transparent substrate 3 made of glass or quartz and a phase shift film pattern 4 formed on the transparent substrate 3. The phase shift film pattern 4 has alternately arranged lines and spaces respectively. On the phase shift film pattern 4, a light shield film pattern 5 comprised of a chromium film is formed. The light shield film pattern 5 has alternately arranged lines and spaces. The line width of the light shield film pattern 5 is smaller than that of the phase shift film pattern 4.
In the phase shift mask 2 having the above-mentioned structure, a 180.degree. phase difference is exhibited between light 1 passing through the phase shift mask pattern 4 and light directly passing through the transparent substrate 3. In other words, electric fields 6 and 7 respectively established by the light passing the phase shift mask pattern 4 and the light directly passing through the transparent substrate 3 exhibit a 180.degree. phase difference, as shown in FIG. 2.
As a result, the light beams reaching a wafer to be patterned interfere with each other as shown in FIG. 3, thereby improving the image contrast. In this case, however, there is a limitation on the depth of focus or the fineness of the pattern when the wavelength of light is constant. It, therefore, is difficult to increase the integration degree of semiconductor devices to a desired level.